1. Field of the Invention
The present invention generally relates to a printed circuit board, and more particularly, to a method of manufacturing a printed circuit organic substrate having metal foils on its opposite surfaces, and the organic substrate manufactured thereby.
2. Description of the Prior Art
Recently following the tendency towards compact size and higher mounting density of electronic appliances, multilayered printed circuit boards have been strongly demanded not only for industrial applications, but also for daily life in general. In the multilayered printed circuit boards as referred to above, a connecting method for effecting an inner-via-hole connection between the circuit patterns in a plurality of layers is required together with a construction at high reliability.
Referring to FIGS. 10(a) to 10(e), a manufacturing method of a conventional two-layered printed circuit board will be described hereinbelow.
In the first place, as shown in FIG. 10(a), through-holes 83 (only one through-hole 83 is shown in each of FIGS. 10(a) to 10(e)) are formed at predetermined places of an insulating substrate 82 such as a glass epoxy substrate or the like, applied with a sheet 81 on its one surface, while, onto the other surface, i.e., an under surface of the substrate 82, a first copper foil 84 is bonded. Then, electro-conductive paste 85 is filled in the through-holes 83 as shown in FIG. 10(b) by printing the sheet 81 as a printing mask. Thereafter, when the sheet 81 is separated from the insulating substrate 82 as shown in FIG. 10(c), the interior of the through-holes 83 is filled with the electro-conductive paste 85. However, there are cases where the electro-conductive paste 85 depending on the thickness of the sheet 81 in amount remains at the upper portion of the through-hole 83 in a state swelling or rising above the surface of the insulating substrate 82. Subsequently, as shown in FIG. 10(d), after applying a second copper foil 86 onto the upper surface of the insulating substrate 82, said substrate 82 and the copper foil 86 are perfectly bonded to each other, and simultaneously, the electro-conductive paste 85 is cured. Then, as shown in FIG. 10(e), the first copper foil 84 and the second copper foil 86 are selectively etched as required, thereby to form a first circuit pattern 87a and a second circuit pattern 87b as illustrated.
In the manner as described above, the first and second circuit patterns 87a and 87b are subjected to the inner-via-hole connection by the electro-conductive paste 85 filled in the through-holes 83, and thus, the two-layer printed circuit board 88 is obtained.
Subsequently, a manufacturing method of a conventional multilayer printed circuit board will be explained with respect to a four-layer printed circuit board as one example by referring to FIGS. 11(a) to 11(d).
In the first place, as shown in FIG. 11(a), an insulating substrate 92 provided with a sheet 91 on its one surface and formed with through-holes 93 at predetermined places thereof is stuck onto the first two-layer printed circuit board 88 prepared through the steps of FIGS. 10(a) to 10(e). Then, as illustrated in FIG. 11(b), electro-conductive paste 94 is filled in the through-holes 93 by printing a sheet 91 as a mask. Thereafter, when the sheet 91 is separated from the insulating substrate 92, the electro-conductive paste 94 is filled only in the through-holes 93 as shown in FIG. 11(c). Subsequently, as shown in FIG. 11(d), by applying and bonding the second two-layer circuit board 95 prepared by the steps similar to those in FIGS. 10(a) to 10(e) and formed with circuit patterns 96a and 96b, onto the insulating substrate 92, the four-layer printed circuit board is obtained.
However, in the conventional arrangements as described so far, there have been problems as follows.
Firstly, in the above known construction, there is a limitation in the content of the electro-conductive substances for the electro-conductive paste which may be used in the printing process for filling said paste into the through-holes, and this fact was unsuitable for reducing the resistance value of the electro-conductive paste, and that between the elecro-conductive paste and the metal foils. More specifically, when the content of the electro-conductive substances in the electro-conductive paste is increased for reducing the resistance value of the electro-conductive paste filled in the through-holes, fluidity of the paste is adversely affected, with consequent lowering of an aptitude for printing, thus tending to give rise to defects such as faulty filling and the like.
Therefore, in order to fill the electro-conductive paste into the through-hole of a small diameter, it is necessary to raise the fluidity of the electro-conductive paste to a certain extent. For the above purpose, the fluidity of the paste may be increased, with an improvement of the aptitude for printing if the content of the electro-conductive substances in the electro-conductive paste is decreased, but in that case, there is such a drawback that the resistance of the electro-conductive paste after curing is increased by an extent in which the content of the electro-conductive substances is reduced.
Secondly, in the conventional arrangements, there are cases where the electro-conductive paste 85 in the amount depending on the thickness of the sheet 81 remains in the state swelling or rising higher than the surface of the insulative substrate 82 as shown in FIG. 10(c). If the second copper foil 86 is applied onto the insulating substrate 82 in the above state, there is no place for the swollen electro-conductive paste 85 to escape, and in some cases, said paste penetrates into a gap between the second copper foil 86 and the insulating substrate 82 as shown in FIG. 12(a). If the second copper foil 86 for such insulating substrate 82 as referred to above is etched to form the second circuit pattern 87b, a short-circuiting path 85a is formed by the electro-conductive paste 85 which has penetrated into between the second copper foil 86 and the insulating substrate 82 as shown in FIG. 12(b), thus resulting in a short-circuiting fault with respect to near-by circuit patterns.
Due to the problems as described so far, the number of the inner-via-hole connections and circuit pattern density which can be formed per unit area are limited in the conventional organic substrates used for the printed circuits, and therefore, it is difficult to realize a multilayer circuit board for high density mounting which will be expanded in demand still more henceforth.